The present invention relates to an improved fixed roof flammable liquid storage tank with a fire extinguishing device.
The improved fixed roof flammable liquid storage tank having a fire extinguishing device of the present invention pertains to the oil and petrochemical industries from the viewpoint of inherent fire safety for storing flammable liquids and the conservation of the petroleum product stored.
The present state-of-the-art for bulk storage of flammable liquids provides tanks which are governed principally by safety requirements and the need to operate economically when the tank is in service. The majority of tanks for the storage of petroleum in bulk are of mild steel welded construction, vertical cylindrical in shape. The tank design, fabrication, site erection, inspection and testing are fully specified in B.S. 2654: Part I, 1965 and Part II, 1961 and in API Standard 650. The British Standard relates to above ground tanks of the following designs:
"Non-pressure" fixed roof tanks (all sizes) suitable for working at atmospheric pressure, but designed for an internal pressure of 66 Kgf/m.sup.2 and a vacuum of 63.5 Kgf/m.sup.2 plus superimposed loads;
"Pressure" fixed roof tanks up to 30 m in diameter, suitable for an internal pressure of 203 Kgf/m.sup.2 and a vacuum of 64.5 Kgf/m.sup.2 plus superimposed loads;
Floating roof tanks of all sizes, i.e. open top tanks;
The "Non-pressure" fixed roof with internal floating deck tanks are used for the storage of both class "A" (flash point below 22.8.degree. C.) and Class "B" (flash point between 22.8.degree. C. and 65.6.degree. C. both inclusive) petroleum;
"Non-Pressure fixed roof with" atmospheric vents, for both Class "B" and Class "C" (flash point above 65.6.degree. C.) petroleum; and
"Pressure" fixed roof tanks for Class "A" petroleum only.
The most common of the presently available storage tanks are:
Supported cone roof: a roof formed to approximately the surface of a right cone, with its principal support provided by either rafters on girders and columns or rafters;
Self-supporting cone roof: a roof supported only at its periphery;
Self-supporting dome roof: a roof formed to approximately a spherical shape, supported only at its periphery; and
Self-supporting umbrella roof: a modified dome roof so formed that any horizontal section is a regular polygon with as many sides as there are roof plates, supported only at its periphery.
All these roofs provide reduction in the loss of flammable vapors. The salient features of the various storage tanks available are compared in Table 1 with what is termed as an "Ideal Tank" wherein each of the five fire safety rating factors receives 20 points or a total fire safety rating of 100.
An ideal tank would be such a pressure vessel in which all venting is eliminated thus containing fully the flammable compound and adequately spaced or otherwise protected to prevent exposure to fire. Such a tank would seem to be economically impractical, but it is used for rating purposes only.
TABLE 1 __________________________________________________________________________ Fire Safety Ratings, conservation Type Tanks Vs. Standard Fixed Cone Roof Tank as per Fire Protection Manual, Second Edition, Edited by Charles H. Vervalin, Gulf Publishing Company, Book Division, Houston, Texas, 1973 Susceptibility Dependence on Dependence Fire S. Susceptibility Susceptibility to fire to damage fire fighting safety on safetyical No. Type of Tank to explosion from venting of vapors from fire exposure efficiency appurtenances rating __________________________________________________________________________ 1. *Ideal Tank 20 20 20 20 20 100 2. *Unprotected 0 0 0 0 20 20 Cone Roof Tank (FIG. 1) 3. *Standard 5 5 5 5 15 35 Cone Roof Tank (FIG. 2) 4. *Variable 20 10 10 10 10 60 Vapor Space Tank With Inert Gas (FIG. 3) 5. Breather Roof 20 15 10 10 5 60 Tank (FIG. 4) 6. Balloon Roof 15 10 15 10 10 60 Tank (FIG. 5) 7. Cone Roof 20 10 15 10 5 60 Tank with Inert Gas and and Water Cooling (FIG. 6) 8. *Lifter Roof 20 10 10 10 10 60 Tank (FIG. 7) 9. *Noded 20 15 15 10 10 70 Spheroid (FIG. 8) 10. *Floating 15 20 10 10 20 75 Diaphragm Tank (FIG. 9) *Pan Type 20 15 20 20 10 85 Floating Roof Tank (FIG. 10) __________________________________________________________________________
The drawbacks of the presently available flammable liquid storage tanks are:
In comparing the various tanks available with the ideal tank, each is penalized to the extent of the apparent existing fire safety problem. In the case of an unprotected cone roof tank as shown in FIG. 1 of the drawings accompanying this specification, the drawback is that it is having inadequate vent devices, no flame arrester and inadequate spacing, and has a fire safety rating of only 20. This is the lowest fire safety rating received by any type of tank by virtue of its fire safety features. Next lowest in fire safety rating is the standard cone roof tank, as shown in FIG. 2 of the drawings accompanying this specification, having a fire safety rating of 35. The major drawback of this standard cone roof tank is that it is highly susceptible to explosion, to fire from venting of vapours, to damage from fire exposure, and is having greater dependence on fire fighting efficiency and for safety on mechanical appurtenances. The variable vapor space tank as shown in FIG. 3 with inert gas protection receives a fire safety rating of 60. The drawback of this variable vapor space tank is that there is a greater possibility of fire from venting of vapors. The variable vapor space tank as depicted in FIG. 3 is considered to be more susceptible to damage by fire exposure but less dependent on mechanical appurtenances than the cone roof tank with inert gas protection as shown in FIG. 6. The breather roof tank as depicted in FIG. 4 receives a fire safety rating of 60; the drawback of this breather roof tank is that since its roof operates by metal bending, it often leads to cracking of the roof sheets, which further adds several drawbacks of its greater susceptibility to damage from fire exposure and greater dependence for fire safety on mechanical appurtenances and on fire fighting efficiency. FIG. 5 shows the balloon roof tank which rates with a fire safety rating of 60 with a major drawback of the cracking of the roof sheets because of metal bending operation. The other drawbacks that follow are its greater susceptibility to fire from venting of vapours because its roof has a larger diameter than the tank which provides greater displacement capacity than its relative breather roof; and higher susceptibility to damage from fire exposure with a greater dependence for safety on mechanical appurtenances and on fire fighting efficiency. The water cooled, inerted cone roof tank as shown in FIG. 6 has a fire safety rating of 60 with a major drawback that there is a greater possibility of fire from venting of vapours. The lifter-roof tank, as depicted in FIG. 7 rates with a fire safety rating of 60 because the possibility of fire from venting of vapours is greater in this tank. The noded spheroid as shown in FIG. 8 is used for storage of oils under pressure, as is required for some casinghead gasolines, butane, and butane blends etc. and is suitable for working pressures up to 1.70 bar. The drawback of this noded spheroid is that it is more dependent for fire safety on mechanical appurtenances and on fire fighting efficiency. Floating diaphragm tank as shown in FIG. 9 rates high with a fire safety rating of 75, as it eliminates practically all vapour losses and is reasonably independent of mechanical appurtenances for safety. The major drawback of the floating diaphragm tank is that there is a possibility of vapour leakage past the diaphragm, which could cause an explosive mixture in the vapour space and has a reasonable susceptibility of the diaphragm to damage from fire exposure, and therefore, has greater independence on fire fighting efficiency. The pan type floating roof tank, as depicted in FIG. 10, has the highest fire safety rating of 85 because the large majority of the fire safety deficiencies involving bulk storage have been eliminated. But its major drawback is its dependence on maintaining a reasonably tight seal because of mechanical appurtenances and reasonable susceptibility to fire from venting of vapours.
The present state-of-the-art of floating roof tanks with emphasis on minimizing seal gaps, as reported by Mr. J. C. Thompson in his article on "Floating Roof Tank Safety Features", states that the environmental requirements should still further improve the fire safety record of floating roof tanks. However, there is a cloud on the horizon, the secondary seal, which has been around for a long time; but in the past, it was usually shoe-mounted with very little vapour space, but now deck-mounted-to-shell secondary seals have much larger enclosed vapour space which pose a threat of greater fire hazards in floating roof tanks in long runs. However, this fire safe tank conserves petroleum product through decreased vapourisation. The gross savings can be applied to amortize the increased cost and defray the increased depreciation and maintenance. FIGS. 1 to 10 thus illustrate the present/prior arts of the types of tanks used for the storage of flammable liquids.
Indian Patent no. 129753 relates to a storage silo construction. This storage silo particularly pertains to the bulk storage of grain and other cereal crops. The details are as shown in FIGS. 11 to 14 of the drawings accompanying this specification.
The storage silo construction is for the storage of grain and other cereal crops, having conical of frusto-conical wall, capable of being fabricated substantially at the ground level, the top of the conical walls being provided with roofing sheets to form a roof of generally conventional design having an opening through which the grain to be stored in the silo is fed by means of an elevator and a conveyor system; the periphery of the roof is being connected to hoisting devices, preferably being electrically operated screw jacks, evenly spaced therearound; the floor of the silo is formed so as to slope upwardly and outwardly from either side of a central valley or trough, details of the trough being seen best in FIG. 13 of the drawings accompanying this specification; the trough comprises a plurality of reinforced concrete channels of U-section located end-to-end in the base of the silo; the floor of the silo being a stabilized earth fill provided with a layer of a premixed anionic bitumen-emulsion and sand, and covered with sheet metal plates welded together, the lowermost panels extending over the sides of the concrete channels so as to form a plurality of hoppers or the like having discharge outlets; each of discharge outlets is provided with a closure plate which can be operated to open and close the discharge outlet as desired; immediately below the discharge outlets runs an endless conveyor belt serving to remove the grain from the silo to a discharge location outside the silo.
The drawbacks of the storage silo construction as described above are as under:
(1) The removable roof of this storage silo construction is of a conventional design and constructed in a conventional manner and particularly suitable for the storage of grain and other cereal crops. With a particular reference to the storage of flammable liquids, the roof is highly susceptible to fire and/or explosion. This is because of the following reasons: PA1 (2) The silo is dependent on external fire fighting arrangements and as such is unsuitable for the storage of flammable liquids. PA1 (3) The bottom of the storage silo construction, being made for the storage of grain and other cereal crops, does not offer enough stability and tightness required for the storage of flammable liquids. PA1 (4) The wall of the conical shell of the storage silo construction, being made for the storage of grain and other cereal crops, is not hydraulically leakproof for the storage of flammable liquids.
(a) The removable roof has electrically operated devices for lifting of the roof. These devices are potential fire hazards as these can cause electrical sparks; PA2 (b) The roof provided is not leakproof to flammable liquid vapours.
With the above noted drawbacks in mind, a tank of the type of the storage silo construction described above if used for storing flammable liquids would have the lowest fire safety rating of only about 20, considering 80 penalty points altogether for susceptibility to fire and explosion, dependence on fire fighting efficiency and electrical/mechanical appurtenances, which from fire safety point of view single it out as totally unsuitable for the storage of flammable liquids.
The present state-of-the-art of extinguishment of oil fires in fixed/floating roof storage tanks implies use of foam and/or dry chemical powder.
The drawbacks associated with the presently available fire extinguishing devices are:
Singly or in combination, foam and/or dry chemical powder fire extinguishants are presently being used for the extinguishment of flammable liquid storage tank fires, either by top surface application or by subsurface/semi-subsurface applications. The drawback of top surface application is that when the roof of the fixed/floating roof tank gets blown off, the top surface application device(s) gets damaged and is unable to perform its function. In case of subsurface/semi-subsurface application, the foam bubbles carry along with them the flammable liquid and thus add to the fire. Moreover, the foam bubbles get broken off due to immediate heat contact and increase in the surface tension of the bubble surface because of the embedded flammable liquid; thus fueling sufficiently further to the fire and ultimately, rendering themselves ineffective in extinguishing the fire; whereas in case of the dry chemical powder, the major drawback is that it can not be applied by using subsurface semi-subsurface application methods.
The prior art discussed in above paragraphs were brought about during the course of time by the necessity for fire safety and conservation of petroleum products, and each of them has drawbacks as detailed above.